Abstract

Within the Hepburn metamorphic-plutonic internal zone of the Wopmay orogen (Northwest Territories, Canada) there are two chronologically and petrologically distinct plutonic associations. The more voluminous of the two, the older 1.885 Ga Hepburn intrusive suite, includes rocks ranging in composition from gabbro to granite, peraluminous granite dominating. The younger 1.855 Ga neighboring Bishop intrusive suite (also gabbro to granite) represents the waning stages of a well-documented calc-alkaline arc, the Great Bear magmatic zone. The petrological distinctions between the two suites are all late-acquired features imposed primarily by contrasting environments of emplacement. Hepburn magmas were intruded within a closing, dominantly sedimentary, back-arc basin. Magma emplacement was synchronous with crustal imbrication, regional metamorphism, and translation of the basin-fill units onto Archean crust. Significant assimilation of sedimentary host rocks by the rising Hepburn magmas occurred, whereas the postregional metamorphism emplacement of the Bishop magmas precluded similar assimilation. The gabbroic contribution observed in the Hepburn intrusive suite is interpreted to reflect a mantle-derived precursor inherited from the back-arc rifting event that immediately preceded emplacement of the suite.